The present inventive concept relates to integrated circuit devices, and more particularly, to image sensors and related fabrication methods.
A CMOS image sensor (CIS) may be used to convert an optical image signal into an electrical signal. A CIS may provide a lower operational voltage and/or reduced power consumption as compared to a charge-coupled device (CCD) image sensor. A CIS may also allow for higher integration, and thus, may be widely used in many fields.
A CIS may include a photodiode formed in a substrate. The photodiode may be configured to accumulate electrons in response to incident light. The CIS may further include an intermediate layer having metal interconnection patterns therein on the substrate, as well as a microlens and a color filter on the intermediate layer. Light may enter the microlens and/or the color filter on the “front side” of the substrate, and may pass through the intermediate layer to the photodiode. Such image sensors are referred to herein as “front-side illuminated” sensors. The metal interconnection patterns are configured to output an electrical signal based on the light incident on the photodiode. However, the incident light may be reflected by the metal interconnection patterns and/or may be absorbed by the intermediate layer, which may adversely affect the sensitivity of the CIS. In particular, when incident light is absorbed by the intermediate layer, the light sensitivity and/or the quantum efficiency of the CIS may be reduced. Also, when light is reflected by the metal interconnection patterns, the reflected light may be received by an adjacent photodiode, which may result in crosstalk problems.
Accordingly, image sensors have been developed where the micro lens and/or color filter are provided on the “back side” or bottom of the substrate, and incident light enters the backside of the substrate. Such image sensors are referred to herein as “backside illuminated” sensors (BIS). In a backside illuminated CMOS image sensor, the substrate including the photodiodes therein and the intermediate layer including the metal interconnection patterns therein may be bonded or otherwise attached to a supporting wafer, such that the substrate including the photodiodes may be opposite the supporting wafer. The entire structure may be turned upside-down (or “flipped”), and the substrate including, the photodiodes may be thinned. The color filter and the microlens may thereby be provided on the bottom or backside surface of the substrate opposite the supporting wafer, such that the photodiodes are adjacent to the color filter and the microlens. However, as photon flux may be dependent upon both the wavelength of the incident light and the depth of the photodiode, light having relatively long wavelengths may pass through photodiodes having relatively shallow depths. For example, for photodiodes having a depth of about 5 μm or less, electrons provided by longer wavelength light may not accumulate into the potential wells of the photodiodes. As a result, the quantum efficiency of the CIS may be deteriorated.